1. Field of the Invention
This invention relates to integrated circuits, and more particularly, to the testing of temperature measurement integrated circuits.
2. Description of the Related Art
It is well known that the voltage drop of a forward biased PN junction varies in a complementary fashion with respect to absolute temperature. For example, in a BJT a change in Vbe relative to a change in absolute temperature may be in the range of −1 to −1.5 millivolts per kelvin and linear to a first order approximation. This relationship may be referred to as complementary to absolute temperature (CTAT). On the other hand, the difference in the value of base-emitter voltage for a transistor operating at a first collector current density, J1, versus the value of Vbe when the transistor is operated at a second collector current density, J2, may be directly proportional to absolute temperature (PTAT).
The relationship between temperature and the difference in the value of base-emitter voltage may be given by: T=q*(Vbe1−Vbe2)/(k*ln(J1/J2))                where k=1.38×10−23, Boltzmans constant                    T=absolute temperature in kelvins            q=1.602×10−19, charge of an electron            J1=the collector current density for Vbe1            J2=the collector current density for Vbe2                        
Temperature measuring devices may be constructed which measure the base-emitter voltage of a transistor operating first at one current density and then at a different current density and then use the above relationship to calculate the corresponding value for temperature. For example, a typical temperature-measuring device based on this principle may use a temperature sensing diode located at the point where the temperature measurement is desired. A first known current may be applied to the diode and the corresponding voltage drop across the diode may be recorded. Immediately subsequently, a second known current may be applied to the diode and the corresponding voltage drop across the diode may again be recorded. Since only a single diode is used in this device, the ratio of the current densities may be the same as the ratio of the applied currents. This ratio along with the two measurements for Vbe may be sufficient to solve the equation given above for absolute temperature, which may then be readily converted into any desired units of measure.
In order to calibrate or determine the accuracy of a temperature measuring device such as the one described above, it may be necessary to place a temperature sensing diode in an environmentally controlled chamber in which the temperature may be accurately measured and controlled. The accuracy with which the temperature of the environment can be controlled and measured may determine the accuracy with which the temperature-measuring device can be calibrated. For example, if an environmental chamber is capable of maintaining an internal temperature to within plus or minus one half degree, the accuracy of the temperature measuring device being calibrated may be somewhat less than this figure.
Also, the amount of time needed for an environmental chamber to reach a steady state temperature and for the device under test to achieve thermal equilibrium with the environment is typically quite large. The time required for a device under test to achieve thermal equilibrium with the environment of a typical thermal chamber after a change has been made to the desired temperature of the chamber may be on the order of several hours. Therefore, determining the accuracy of a temperature-measuring device at many points over a wide range of temperatures may be a very time-consuming undertaking.